The study, published in the Feb. 4 issue of the Journal of the American Medical Association, comes a day after the American Heart Association urged that CT scans be used judiciously to minimize exposure to ionizing radiation. That recommendation came in an advisory published in Circulation.

"It is a coincidence," said Dr. Thomas C. Gerber, an associate professor of medicine and radiology at the Mayo Clinic in Jacksonville, Fla. "These projects have been going on for months or years. The timing ultimately was a coincidence." Gerber was an author of the study and lead author of the advisory.

The two reports show the intense interest in 64-slice CT scans, so called because they produce that many simultaneous images of the heart. A CT scan gives a radiation dose equivalent to 600 chest X-rays.

The Circulation report noted that medical imaging using X-rays accounted for half the medical radiation dose to which people in the United States were exposed in 2006, and that its use was growing by 5% to 10% a year.

The JAMA report found that the radiation exposure of 1,965 people having 64-slice cardiac CT scans at 50 medical centers in a number of countries, including the United States, varied more than sixfold. Radiation doses ranged from 331 mGy x cm (a measure of absorbed radiation) to 2,146 mGy cm.

Sloppiness was not the cause of the wide range of exposure seen in the study, Gerber said. "We actually thought it would be wider," he said. "It has nothing to do with technician competence. It's that there is not one protocol of radiation setting that applies to all patients."

The amount of radiation depends on such factors as what is being scanned, Gerber said -- all or part of the heart. It can also depend on the heart rate of the person being scanned, the regularity of the heart rate and other factors.

Obesity is one of those factors, said Dr. Andrew J. Einstein, an assistant professor of clinical medicine in radiology at Columbia University, who wrote an editorial accompanying the JAMA report.

Still, Einstein said, "some sites are more concerned and more careful about radiation exposure, and attention must be paid to reducing the radiation dose."

But one factor that makes the exposure report less ominous is its timing, he said. The study ended in 2007, just as a lower-radiation technique called sequential scanning was being introduced. Sequential scanning can reduce radiation exposure by 78% and is being widely adopted, Einstein said.

CT scanning "is a technology that is still finding its place in the constellation of diagnostic methods," he said. "It seems that it is especially good at excluding coronary disease. We can visualize the coronary arteries without doing an invasive procedure."

CT scans can be very useful in assessing the condition of someone who shows up in an emergency room complaining of vague chest pains, Einstein said. "The emergency room doctor dose not feel certain about sending such patients home without a test," he said. Yet doctors are reluctant to do an invasive test, such as angiography, which sends a probe into the heart and has its own dangers, Einstein said.

The same is true of someone seen in a doctor's office with "chest pain of an etiology which remains unclear," Gerber said. "CT angiography tends to be very valuable for patients who have symptoms or are at high risk for coronary artery disease."

It is not valuable for people who are not at high risk and have no symptoms, Einstein said. "We don't think it should be used for screening purposes," he said.

Gerber said that worries about increased risk of cancer because of radiation exposure should be seen in the context of overall risk of death. "The people who benefit most from these scans are those in the older age range," he said. "Cancers tend to take 10 to 40 years to manifest themselves. Without these scans, they might never have lived to see the cancer."

What is missing thus far are studies of the risk-benefit balance of CT scans, both Einstein and Gerber said.

"Most of us understand that the benefits have never been demonstrated in such a study," Einstein said. "There are a number of proposals that have been submitted to the National Institutes of Health for such studies."